In the case of electromechanical actuators, incremental sensor systems are generally used to determine the speed and direction of rotation, essentially comprising a rotor-side sensor wheel and a stator-resistant arrangement of detectors. The detectors scan the sensor wheel and generate an electrical signal as a function of the rotor position, comprising a series of pulses. This pulse signal is fed to an evaluation/control device for evaluation. The evaluation/control device generally comprises a microcomputer, which determines the path traveled by counting the incremental sensor pulses or calculates the instantaneous speed of rotation from the current length of the rectangular pulses. Where two detectors are used, the direction of rotation can simply be obtained from the phase displacement between these two rectangular signals.
However electromechanically operated actuators for window lifting systems and sliding roofs are known from motor vehicle technology, wherein only a single Hall sensor is used to detect the magnetic field of a rotor-side magnetic sensor wheel.
It is also known that in the case of electromechanical window lifting systems or sliding roof drives, anti-trap protection is required for safety reasons. New legal provisions require a high degree of safety. If trapping occurs, not only does the movement of the closing element have to be stopped but once a maximum permitted trapping force has been exceeded, the drive has to be reversed immediately and the closing element has to be moved back in the opening direction. Because of the mechanical inertia of the drive system however the actual reversal of the direction of rotation does not correspond to the electrical switching process, the reversal of the polarity of the armature voltage of the motor but follows this with a time delay. Effective anti-trap protection is then only ensured, if the control device can detect the actual reversal of the direction of rotation correctly.
In order also to be able to identify this actual reversal of the direction of rotation in the case of an arrangement with just one magnetic field detector, a magnetic wheel is proposed in EP 1175598 B1, having magnetic poles disposed asymmetrically along the periphery. It has nine poles of equal pole width and a reference pole with a pole width that is comparatively larger than this. With this arrangement the problem can arise that a measurement uncertainty results, if the drive is randomly reversed from a position, in which the reference pole is located spatially in a position diametrically opposite the Hall sensor. As the pattern of the pulse diagram has mirror symmetry at this reversal point, the determination of the actual reversal of direction of rotation can be subject to an error of plus/minus half a rotation of the drive shaft. In the case of a window lifting drive this can result in the safety effect of the anti-trip protection being impaired or the window not fully achieving the intended closing position.